The invention is in the field of nuclear magnetic resonance (NMR) and particularly relates to NMR probes having superconducting components.
NMR data are among the most precise characterizations of the chemical structure of matter, being based upon time/frequency measurements. The sensitivity, R, of an NMR instrument is limited by the relationship of the signal amplitude to the noise in the signal channel. Where the noise is treated as the rms value of the observed noise level, the sensitivity may be expressed as
R=S/(2Nrms)
and S/Nrms, the signal to noise parameter can be shown to be directly proportional to the square root of the quality factor, Q, of the receiver probe coil and inversely proportional to the square root of the temperature characterizing the probe coil resistance.
Thermal adjustment of the RF circuit of an NMR probe has been known for a number of years. NMR probe sensitivity is significantly enhanced by reduction of thermal noise by cooling of normal metal NMR coils as demonstrated by Styles, et al, J. Mag. Res. v. 60, 397 (1984) to yield a sensitivity gain of about a factor of three compared with a conventional probe. Rollwitz, U.S. Pat. No. 3,764,892, proposed a superconducting NMR coil, but this apparatus was not reduced to practice. Black, et al describe an NMR imaging probe using high temperature superconducting (HTS) materials. The application of these materials to high resolution NMR has been described by Hill, IEEE Trans. Appl. Superconductivity, v.7, p. 3750 (1997) and by Anderson, et al, Bull. Magn. Reson., v17, p.98 (1995).
The sensitivity of an NMR probe for fixed sample volume is also proportional to xcex7, the filling factor, for the coil. Thus, one may manipulate xcex7, Q and T to improve sensitivity. In particular, superconducting coils may achieve a Q of the order of 104 compared to values less than 102 for conventional coils. In addition, superconducting coils operate at temperatures of about 30xc2x0 K. compared to 300xc2x0 K. Thermal control of the NMR probe coil necessitates provision for thermal isolation between the coil and the object under study. As a consequence, the filling factor for the coil is diminished. With allowance for loss in filling factor appropriate to a typical 5 mm sample tube, sensitivity enhancement of an order of magnitude is achievable in respect to the conventional NMR probe. Clearly, a coil operating at room temperature facilitates maximization of filling factor.
The transmit (excitation) pulse of an analytic NMR instrument must produce an RF field of sufficient magnitude to excite all resonances in the frequency range under study. It is often desirable to produce and/or detect resonance conditions concurrently in different nuclei and this function is accommodated in an RF probe circuit capable of supporting resonance at multiple frequencies. Such multi-resonant RF probes are well known in the art. Generally, such a probe circuit supports two separate RF channels, each independently tunable where the two channels incorporate a common inductance coupled to the sample. One practical attribute of multi-resonant circuits is that the sensitivity in either channel is rather less than might be achieved in a corresponding singly resonant probe circuit. Moreover, in a multiply resonant circuit, the sensitivity can only be optimized at a one of the resonant frequencies while the remaining resonance(s) result in circuit sensitivities(s) significantly less than the circuit sensitivity for that one resonance. Accordingly, it would be desirable to increase the sensitivities of each RF channel of a multi-resonant NMR probe to approach the sensitivity achievable for corresponding singly resonant probe circuits.
In the prior art, a balanced dual resonant circuit is known wherein the high Q value properties of coaxial transmission lines is exploited to effectuate trap inductances. See U.S. Pat. No. 4,833,412, commonly assigned herewith.
An object for the present invention is the achievement of a multiply resonant NMR probe circuit wherein the sensitivity of the RF channels more closely approaches the sensitivity characterizing an equivalent singly tuned circuit for any such channel.
Further and additional advantages and objectives of the invention will be set forth in the description which follows, and in part will be apparent from practice of the invention.
The objectives are achieved in a mutliply tuned circuit which is realized from a first resonant circuit which couples to the object under study (typically via inductive component, or sample coil surrounding said object) and which first circuit communicates with at least one other resonant circuit (including a trap coil) which is remote from the object, and thus from the inductive component of the first circuit. Each of these first and other circuits supports an RF channel for communication to either a corresponding RF source or for the processing of a resonant signal developed in the respective circuit. The two resonant circuits preferably communicate through simple series connection whereby the RF channel which is directed to the other, or trap circuit (usually a low frequency channel) encounters a series connection to ground. The higher frequency RF channel is directed in parallel to the two resonant circuits. As described this multiple tuned circuit is conventional. However, as shown below, the sensitivity is relatively degraded for the two RF channels as a function of the ratio of the Q values for the respective inductive components. If the trap coil and/or components of this resonant circuit (which is remote from the object of study) incorporates superconductor in the superconducting phase, the corresponding Q value becomes very large, e.g. of order 103 to 104, whereas the non-superconductive resonant circuit coupled to the object retains a Q value of the order 102. The sensitivity of the respective RF channels may be adjusted to more nearly correspond to the sensitivity of independent singly tuned resonant circuits by the employment of superconductor in only one sub-circuit of the dual resonant circuit.